Field of the Invention
The present invention relates to a switching power conversion circuit providing a PFC (power factor correction) function with low THD (total harmonic distortion) while converting a standard AC (alternating current) voltage or a phase-cut AC voltage to a DC (direct current) output voltage or a DC output current.
Description of the Related Art
Conventional switching power conversion circuits are generally used to convert an AC voltage source to a DC output voltage or a DC output current for a load, and the load can be an LED (light emitting diode) module, a battery or an electronic device, etc.
Besides, when converting an AC voltage source to a DC output voltage or a DC output current, a good power factor is generally required. Taking a buck-boost power converter with a constant on-time mechanism operating in a BCM (boundary conduction mode) for example (the major operation waveforms thereof are shown in FIG. 1), the operation principle thereof can be characterized by the equations as follows:IPK=(VIN/LMAIN)*TON,  (1)where VIN is a full-wave rectified voltage derived from a standard AC voltage; LMAIN is an inductance value of a main inductor for storing and releasing energy in terms of current; TON is a turn-on time for building up a current flowing through the main inductor in each cycle of the switching cycles, and the switching cycles have different periods corresponding to different levels of the full-wave rectified voltage VIN present at different start time points of the switching cycles; and IPK is a peak value of the current through the main inductor built up per switching cycle, and IPK has different values corresponding to different levels of VIN respectively.TOFF=(VIN/VOUT)*TON,  (2)
where VOUT is a DC output voltage, TOFF is a turn-off time in each switching cycle for discharging the current through the main inductor, and the length of TOFF is dependent on the corresponding voltage level of VIN. In general, there are many switching cycles during a half sinusoidal period of VIN, each switching cycle corresponds to a different level of VIN, and the higher/lower the level of VIN is, the longer/shorter the length of TOFF will be. That is, when VIN is at higher/lower levels, IPK will get higher/lower, and it will take longer/shorter time to discharge the current through the main inductor.
                                          (            3            )                    ⁢                      I                          IN              ⁢                              -                            ⁢              AVG                                      =                ⁢                              (                          1              /              2                        )                    *          Ton          *                                    I              PK                        /                          (                                                T                  ON                                +                                  T                  OFF                                            )                                                              =                ⁢                              (                          1              /              2                        )                    *          Ton          *                      [                                          V                OUT                            /                              (                                                      L                    MAIN                                    *                                      (                                                                  V                        IN                                            +                                              V                        OUT                                                              )                                                  )                                      ]                    *                      V            IN                                                            =                    ⁢                      G            *                          V              IN                                      ,            
where IIN-AVG is an averaged input current per switching cycle, and G=(½)*Ton*[VOUT/(LMAIN*(VIN+VOUT))] is not a constant gain but inversely proportional to VIN, given fixed values of LMAIN and VOUT. That is, when VIN is at higher/lower levels, G will become lower/higher to introduce a THD into the input current.
To reduce the THD of the input current, one solution is using a sampling circuit to periodically sample the voltage of VIN, and use the sampled voltage of VIN to modify the value of the gain G so as to keep the envelope of the input current as close to the envelope of VIN as possible. However, this kind of solutions generally makes the control mechanism of the switching power converter more complex.
To solve the foregoing problems, a novel switching power conversion circuit is needed.